I. Field of the Invention:
The present invention relates to an emergency lighting circuit for operating a gaseous discharge lamp from an auxiliary DC source upon failure of AC line voltage, and more particularly, to such an emergency lighting circuit improved to provide a transformer coupling the circuit with an AC line source, and which transformer includes a secondary winding and associated circuitry for performing a plurality of circuit functions in a substantially non-interacting manner.
II. Description of the Prior Art:
U.S. Pat. No. 3,906,243 - Herzog, assigned to the assignee of the present invention, discloses a retrofit emergemcy lighting system having means for reducing capacitive leakage current and inductive shunt current losses. One benefit thereof is the substantial reduction of the electric shock hazard possibly resulting from replacing a lamp during high frequency inverter circuit operation. However, the Herzog '243 patent discloses a lag ballast circuit for 60 Hz operation which inherently produces inefficient operation of a lamp, yielding light output of about 50% of lamp capability, in addition to having a low input power factor.
An improved circuit is disclosed in Application Ser. No. 718,028 filed Aug. 26, 1976, assigned to the assignee of the present invention and filed concurrently herewith. Therein is shown a lead ballast circuit for 60 Hz operation which produces higher light output using the same magnetics as in the Herzog 3,906,243 circuit. In a two lamp circuit, as shown therein, it is necessary to provide three filament heater windings. It is possible, in the Alley circuit, to place two of these windings over the secondary coil. There is, however, no room to place the third filament winding thereon. It was found that, when using an emergency lighting arrangement as shown in U.S. Pat. No. 3,921,005 -Watrous, (assigned to the assignee of the present invention), this third filament winding could be placed on the transformer T3 provided therein for battery charging and inverter hold-off or inhibit functions. This placement of the third filament winding on transformer T3 could be accomplished only if the secondary winding S thereon could be reduced in size. As shown in Watrous - 3,921,005, secondary winding S is center tapped to provide, during one half cycle of the AC voltage, charging current for battery 14 and during the other half cycle of the AC voltage, to provide half-wave current for the indicator lamp Pl and, also, to provide a signal to the first sensor at terminal 17 of control circuit 20 for monitoring AC source voltage thereby to operate an inverter hold-off or inhibit function. However, in an improvement on the Watrous circuit, it was found necessary to provide two separate secondary windings on transformer T3 to properly provide these functions, even though in Watrous, winding S was additive center-tapped.
It was found desirable therefore to provide room on an already overcrowded transformer for a third filament winding. Accordingly, it is an object of the present invention to provide space on the transformer for such a third filament winding by combining the functions of the hold-off or inhibit winding and the battery charging winding into a single secondary winding supported by necessary circuit components to prevent interaction of the circuit functions.